The present invention relates to a digital video signal processing device and more particularly to a TV camera device which is arranged to use the processing device.
In general, an imaging device such as a TV camera provides a signal output level defined in a specified range from the maximum to the minimum level. In some circumstances, however, a video signal level may go higher or lower than the specified range. For example, as mentioned below, when an image is picked up under a large amount of incident light, the video signal level goes higher. On the other hand, when it is done under a small amount of incident light, the video signal level goes lower. Or, a contour emphasis treatment is executed in the device. To overcome this drawback, the imaging device has a circuit for clipping a signal of higher or lower signal level than the specified range for suppressing the output signal level inside of the specified range. This circuit is generally referred to as a white clipping circuit or a black clipping circuit.
To digitize a video signal when processing it, the similar clipping means is required to suppress the output signal level inside of the specified range.
FIG. 1 shows a conventional arrangement of a TV camera with processing a digital video signal.
In FIG. 1, numerals 1 to 3 denote an image pickup element. Numerals 4 to 6 denote an analog signal processing circuit, which performs necessary signal treatments such as signal amplification and band restriction with respect to R, G and B video signals picked up from the image pickup elements 1 to 3. Numerals 7 to 9 denote an analog-to-digital converter, which operates to convert the output of the analog signal processing circuit 4, 5 or 6 into a digital signal. The digitized R, G and B signals outputted from the analog-to-digital converters 7-9 are sent to a matrix circuit 10 in which those signals are mixed at a predetermined ratio. Then, the mixed signal is inputted to a contour emphasis circuit 11 in which a contour emphasis signal 12 is produced from the mixed signal. A contour emphasis signal 12 higher than the predetermined signal level is clipped by a clipping circuit 120 which may be provided if necessary. Then, the clipped signal is added to each of the R, G and B signals in an adding circuit 13, 14 or 15. The R, G and B signals to which the contour emphasis signal 12 is added are processed in a signal processing circuit 16, 17 or 18. This processing is gamma correction or the like. The outputs of the signal processing circuits 16 to 18 are sent to clipping circuits 28' to 30', respectively. Those clipping circuits 28' to 30' operate to clip the signals deviated from the specified range and output the clipped signals as Rout 37', Gout 38' and Bout 39', respectively. Each signal is sent to a matrix circuit (not shown) in which the clipped signals are converted into a luminance signal Y and color difference signals I and Q.
However, the foregoing clipping brings about aliasing distortions, the cause of which will be discussed below.
Later, the description will be oriented to why this aliasing distortion takes place with reference to FIGS. 2(A), 2(B) and 3. FIGS. 2(A), 2(B) and 3 are views for explaining why the aliasing distortion is caused by digital signal processing.
In general, to convert an analog signal into a digital signal, the continuous analog signal is sampled at fixed intervals. Hence, it is to be noted that when the analog video signal is digitized, this digitizing may bring about the aliasing distortions, that is, the low frequency portions converted from higher harmonic components. The occurrence of such aliasing distortions are allowed to be prevented by restricting frequency band of an analog signal within a half or less sampling frequency before sampling. This is a well-known Nyquist condition.
As shown in FIG. 2(A), however, if a signal to be sampled (that is, an original analog signal) is a sine wave signal of such a frequency fs as satisfying the Nyquist condition before clipping, the clipping of the signal to keep the predetermined signal level results in causing the clipped signal to be mixed with higher harmonic components of an integer (n) time greater frequency than the original signal, that is, a frequency n.fs as shown in FIG. 2(B). At a time, if the clipped signal containing higher harmonic components is sampled without any band restriction, higher harmonic components having a higher frequency than a half of a sampling frequency fc are converted into low-frequency portions, which conversion brings about the aliasing distortion.
In a case that, on the other hand, the sampled signal (without any band restriction), that is, the digitized video signal which is not operated to clip with analog signal processing is clipped with digital signal processing, this is equivalent to sampling of the signal which is operated to clip with analog processing without any band restriction as described above, which thus brings about the aliasing distortion. This will be understood that the value of each sampling point is converted into a value at the clipping level as shown in FIG. 3.
Hence, if a subject analog signal the frequency band of which is restricted to a half or less of a sampling frequency is sampled, and if the sampled signal (that is, digitized signal) is clipped with digital signal processing, this brings about the aliasing distortion as mentioned above.
For a TV camera system required to do the above-mentioned clipping process, a technique for preventing occurrence of the aliasing distortion by utilizing a process of interpolation as described in JP-A-4-152779 has been known. The term "interpolation" used throughout this specification means a pseudo-interpolation wherein the calculation operation is made at each of intermediate points in a finite number between sampling points.
Later, the concrete arrangement of the technique will be briefly described with reference to the drawings.
FIG. 4 shows a block arrangement in which the technique disclosed in JP-A-4-152779 is applied to a signal level restricting circuit corresponding to the clipping circuit of FIG. 1 included in the signal processing section of a TV camera.
Below, the arrangements and the operations of the signal level restricting circuits 19' to 21' will be discussed in detail.
Data interpolating circuits 25 to 27 at a first stage will be described with reference to FIG. 5 which shows the relation between the original input data, that is, the data to be interpolated and the interpolation data.
Each of the data interpolating circuits 25 to 27 performs an interpolating process with respect to an input signal (referred to as original data signal) sampled from an original analog signal at periods T indicated by X marks of FIG. 5, for generating interpolation data indicated by circles of FIG. 5 located at a middle point of each sampling period T of the original data, for example. This process of interpolation offers an up-converted and interpolated signal having twice as high a rate as the original data signal, that is, a signal sampled at a half of the period T, which signal is composed of the original data signal and the interpolation data signal.
Ideally, an interpolation data value given in the data interpolating circuits 25 to 27 can be presumed by calculating an output provided by means that a delta function series signal passes through an ideal low-pass filter having the Nyquist frequency corresponding to a half of the sampling period T. A delta function series signal represents a discrete sampled signal. This presumption, however, requires an enormous amount of data. Actually, hence, a finite degree digital filter is used for presuming the value with doing a predicting coding process. This finite degree digital filter is arranged of delay elements 41, multipliers 42 and an adder 43 in FIG. 6.
Each of the filter coefficient values C0 to C.sub.2n-1 given to the multiplier 42 may be a designed value of a transversal type digital filter having a half of a sampling frequency fc as a cut-off frequency. As a simple example, in the case of n=2, C1 or C2=0.625 and C0 or C3=-0.125 are given as the filter coefficients.
In order to realize the process of up-converting the data signal in the data interpolating circuits 25 to 27, as shown in FIG. 6, it is just possible to synchronize the original data and the interpolation data with the sampling period, and alternately output these two data at a half of the sampling period by using a switch 44.
The outputs of the data interpolating circuits 25 to 27 are applied to the clipping circuits 28 to 30 in which the signals staying higher or lower than the specified signal level are clipped within the predetermined signal level.
The clipping of the signals in the clipping circuits 28 to 30 causes higher harmonic components to be generated. The frequency of the higher harmonic components in a range of a half of or the same as the (original) sampling frequency before up-converting is made to be in a range of a half or less time as large a value as the up-converted sampling frequency, that is, doubled sampling frequency. Hence, the higher harmonic components generated in the frequency range are not made to be the aliasing distortion while the signal is up-converted as mentioned above.
Further, these clipped signals are filtered by low-pass filters 31 to 33 for removing the higher harmonic components. The low-pass filter has a cut-off frequency set as a quarter of the up-converted sampling frequency and a half of the sampling frequency before the up-conversion.
Each output of the low-pass filters 31 to 33 is applied to the corresponding one of thinning circuits 34 to 36 in which every other piece of data is thinned out for returning the up-converted signal into a digital signal having the same sampling rate as the original data signal. Then, the digital signal is outputted to the later stage (not shown).
As set forth above, this prior art arranged to use the technique disclosed in JP-A-4-152779 operates to clip the digital signal up-converted by an interpolating process. If this clipping process brings about the higher harmonic components, these higher harmonic components fill the foregoing Nyquist condition, and no aliasing distortion takes place with respect to them. This means that this type of harmonic components the frequency of which are higher than of a data signal are allowed to be removed by a low-pass filter without missing the data signal. Thus, the digital signal processing device enables to clip a digital signal without having to bring about any aliasing distortion.
In the signal level restricting circuits 19' to 21' included in this prior art, however, digital circuits of the imaging device in the interval between the up conversion done by the data interpolating circuits 25 to 27 and the thinning-out operation done by the thinning circuits 34 to 36 are required to operate at twice as large an operating frequency as the sampling frequency before the up conversion. This may bring about a significant shortcoming in light of cost, power consumption and stability of operation.
According to another conventional technique, the contour emphasis signal 12 outputted from the contour emphasis circuit 11 is restricted in level through the effect of the circuits equivalent to the signal level restricting circuits 19' to 21'.